The knee joint is one of the most complicated joints in the human body. The tibial and femoral motions involve various translations and rotations, due to leg movements in abduction/adduction, internal/external rotations, and flexion/extension.
When injury occurs to the knee or when diseases as osteoarthrosis set in, reliable diagnosis and treatment evaluation are necessary. For that, functional evaluation of the knee in a three-dimensional manner is essential.
Sophisticated methods exist to measure the functional state of the knee in a dynamic manner. However, these methods are not precise enough to allow the measurement of small angles such as abduction/adduction, and internal/external tibial rotation. Small displacements are tainted by the noise caused by soft tissue movements relative to the bones.
Some have dealt with the problem of skin movement by inserting cortical pins into the bones. With this method, they were able to precisely measure angles in the frontal and transverse planes during gait. However, such methods cannot be widely used in a clinical environment. Some knee braces and harnesses have been developed in order to get 3-D representations of the knee kinematics in a non-invasive manner. For instance, International publication No. WO 01/32080 A1 describes a system for the precise measurement of 3-D knee kinematics.
After a movement has been measured precisely and reliably, it is necessary to represent it in a meaningful way. The knee is not a hinge, and movement about that joint does not occur in a 2D plane. Therefore, it is difficult to represent knee kinematics in a reproducible way.
To improve accuracy and repeatability, most investigators use x-rays to build coordinate systems. This involves radiation for the subject and is not practical because of the need to have access to radiological equipment and to a technician.